Crashing Electrons Could Explain Earth's Magnetic Field Mystery

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A messy paradox that has plagued geoscientists who study Earth's
core and the magnetic field it produces may now be solved.

The puzzle is only a few years old. It was raised in a 2012 paper
in which geophysicists in the United Kingdom published a widely
accepted supercomputer model that found
Earth's iron core was incredibly efficient at conducting
heat. In conduction, heat moves, but the material transferring
the heat stays still — think of a kitchen pan warming up. The
transfer of the heat from the stovetop to the pan is
conduction.

In that study, the researchers examined how heat may move through
the Earth's core, at the level of atoms and electrons. Put
simply, the paradox is that in this model, so much heat escaped
from the core via
conduction that there wasn't enough energy left over to fuel
convection (when heat creates motion) in the liquid outer core.
The implication: Earth's magnetic field shouldn't exist. (If
kitchen pans were as effective at conducting heat as the core,
then meat would never cook because all the heat would escape into
the air.)

"The study attracted a lot of attention because of the serious
consequences," said Bruce Buffett, a geophysicist at the
University of California, Berkeley, who was not involved in the
research.

But now, new research finds that inside the deep Earth, where
temperatures can match those on the surface of the sun, iron's
electrons move heat by more means than just the usual way,
through rapid vibrations, according to a study published today
(Jan. 28) in the journal Nature. Electrons also bash into one
another, transferring energy through collisions known as
electron-electron scattering. [ What Is
Earth Made Of?]

The results resolve the paradox, the researchers concluded in the
new paper. "There was a big problem in how you generate a
magnetic field, and now, because of our results, that problem has
basically gone away," said study co-author Ron Cohen, a staff
scientist at the Carnegie Institution for Science in Washington,
D.C., and a professor at University College London in the United
Kingdom.

Lead study author Peng Zhang, also of the Carnegie Institution,
used a National Science Foundation supercomputer to calculate how
iron 's
electrons zip and zwing within the core. The modeling work is
akin to predicting to position of every water droplet in a rain
cloud, Cohen said. "We're worrying about where every single
electron is, and how they interact and scatter and bounce off
each other," Cohen told Live Science.

The Earth's inner core is solid, and about the size of the moon.
The outer core is liquid; about 1,400 miles (2,250 kilometers)
thick; and topped by 1,800 miles (2,900 km) of crystalline mantle
that flows like warm plastic. This is all encased in a cold, hard
shell of rock called the crust. The core
is not pure iron metal — elements such as oxygen, carbon and
nickel are also present.

Zhang's team discovered that in the core, collisions between
electrons are as important as collisions between electrons and
vibrating atoms (known as electron-phonon scattering) when it
comes to heat energy. The earlier modeling work,
also published in Nature, had concluded that the Earth's core
is losing two to three times as much heat to conduction than
previously thought. Zhang's new findings put the amount of lost
heat back in line with conventional models (because accounting
for the electron-electron collisions gives iron a lower
conductivity).

"These calculations are difficult, as are the experiments, but
confirmation of these results will be important," said Dave
Stevenson, a geophysicist at the California Institute of
Technology who was not involved in either study. However, he
said, it is not yet clear that the new results overturn the
earlier findings from 2012.

"Science is never that simple," Stevenson said. And the new study
won't solve all the questions that remain, such as how the Earth
actually cooled throughout its history, Stevenson said.

Protecting the planet

Since the 2012 model was published, geoscientists have come up
with alternate explanations for how
Earth's magnetic field may work,under the premise that most
heat was escaping through conduction. The planet's magnetic field
has existed for at least 3.4 billion years, according to magnetic
minerals in ancient rocks.

Convection is when heat creates motion. Heat from below causes
material to rise, and as the material cools, it falls back down
again — just like you see in a pot of boiling water or when all
the hot air in a room collects near the ceiling. Scientists think
that convection currents in the core's liquid metal may flow in
spirals due to Earth's constant rotation. The spiraling metal
generates the planet's magnetic field. Without a magnetic field,
Earth would have no protection from the the solar wind, and life
as we know it wouldn't exist. [ Photo
Timeline: How the Earth Formed ]

One alternate way to explain the magnetic field, that doesn't
require heat-driven convection, holds that the convection is
driven by changes in composition inside of the Earth. The
inner core started forming about 1 billion years ago, when
temperatures finally dropped low enough for iron metal to freeze
solid, scientists think. As iron continues to solidify, lighter
elements in the metal mixture, such as oxygen and carbon, may
escape and rise toward the mantle, fueling convection currents.

It's also possible that a heat-driven magnetic field, or
geodynamo, existed before the inner core formed, said Monica
Pozzo, a geophysicist at University College London and leader of
the 2012 modeling work.

"A sure impact of this [new] work will be to intensify the
current debate on the thermal history of the Earth and the
workings of the geodynamo," Pozzo said.